MK never gets old in my opinion, and if it's your/their first trip definitely start there!
As far as crowds... due to me running the PHM and the BF's schedule we often end up there for at least part of President's Day week. I always fret about it in advance and then it's always fine. Like, noticeably busier than the time we went in May, but not impossible. I will say, park hoppers are totally worth the extra money if you can swing it, for a variety of reasons but there was definitely one time we walked into MK and it WAS too many people and we walked out again and went to EPCOT and it was fine. (that's also easier for two adults than for a family with a kid, I acknowledge.)
I don't feel qualified to comment on HS w/ a 4-yo. I think you'd be doing more shows than rides, considering the rides they have there.